The over-all objectives of our work are: (1) to provide some understanding of the nature of the interaction of phosphofructokinase (PFK) with metabolites that regulate the activity of this key enzyme, (2) to determine what differences exist in the regulatory as well as structural properties of PFK in tissues of varying metabolic functions, and (3) to determine the role of the enzyme in regulating carbohydrate metabolism in various tissues. Recently we have developed a procedure that allows purification of the A2B2 hybrid of PFK, the predominant isozyme species of adipose tissue. The regulatory properties of this isozyme will be analyzed. Another recent development is the evolution of a procedure which allows specific chemical modification of PFK at the citrate interaction site. Studies of this modified form will provide further insights into the mechanism of regulation of the enzyme activity.